Logging metrics is not fully correct

[runk]

1. When tags are empty code is still sending them as "empty" chunk:

https://github.com/marceloboeira/aws-lambda-datadog/blob/master/src/logger.js#L6

So ideally there wouldn't be |# part at all.

2. Time should be send as seconds, not microseconds: https://docs.datadoghq.com/integrations/amazon_lambda/?tab=awsconsole#deprecated-using-cloudwatch-logs